Well, it's been shown that there are at least a few combinations that are in game- half draenei, half orc; half human, half elf; and half orc, half ogre; off the top of my head.

I would venture to guess that most of the humanoid races can breed with each other, but that, like with most mixed-species offspring (think ligers, zebroids, or mules), would be sterile and therefore not a species in their own right.

Then you have animals like cats and dogs. I personally believe that it is more seen as "wrong" to mix blood than it is because the offspring is steril.

Well none of them are closely related enough for it to be realistically plausible, except possibly trolls and elves. There are half elves in lore (because it's fantasy so there HAS to be half-elves), even though the revelations about the origins of humans makes that hard to believe. Their closest common ancestor is rocks.